Closer Than You Think?

The small circular object is a false-color, Hubble Space Telescope image of brown dwarf "Gliese 229B." While dozens of times more massive than Jupiter, it is a failed star 100,000 times dimmer than our Sun. It orbits the red dwarf Gliese 229: that star is not in the image, but its light flares along the left frame and and the linear diffraction spike. The brown dwarf orbits Gliese 229 at a distance comparable to that of Pluto from our Sun. These stars are 19 light-years from Earth. Credit: S. Kulkarni (Caltech), D.Golimowski (JHU) and NASA

Every amateur astronomer knows that Proxima Centauri -- of the Alpha Centauri multi-star system -- is the closest star to our Sun. We've known this fact since the early 20th Century. But like a lot of established "facts" in astronomy, this may change: conservative estimates say Proxima's status has a 10-percent chance of being revoked in the next few years.

Astronomers are searching out and finding the distance to dim red dwarf stars that may have been overlooked in previous assays or our galaxy. They are also looking for brown dwarf "stars" -- heat-emitting bodies not large enough to ignite stellar hydrogen fusion. These targeted searches may locate one of these objects in the neighborhood of our Sun, closer than Proxima's 4.3-light years; it may even be a distant and faint binary companion to our Sun. Yes, I said "companion" as in the Sun being part of a binary system!

A solar companion is not so strange when you consider that over half of all stars are in binary systems. While not widely accepted, the companion theory does have some prominent supporters -- one group has even dubbed the companion star "Nemesis." Nemesis could be a red dwarf that has already been cataloged but has not had its distance measured. At only 1 to 3 light-years from the Sun, it could be visible right now in binoculars or a small telescope! If you are still very skeptical of this idea, consider Proxima Centauri's relation with its much larger primary: Alpha Centauri A. Proxima is so dim and far from its primary that it takes over a million years to complete one orbit -- all the while looking like an ordinary field star.

At any rate, whether a solar companion exists or not, detecting red and brown dwarfs closer to us than Proxima Centauri is a real possibility. An accurate count of these bodies will result in better models of the composition of our galaxy. Using the parallax technique -- observing position shifts as the Earth orbits the Sun -- a team from Georgia State University has recently identified 13 unknown stars -- including one 12-light years away that ranks as the 20th closest star to our Sun. There's another reason for locating stars near our Solar System, according to Georgia State researcher Todd Henry: "It's my bet that the first Earth-like planet will be found around a nearby red dwarf."

More information:
AAAS; Jan. 14, 2002; Science News Service: "Star Search for Nearby Neighbors" 

Space.com; Apr. 3, 2001; Solar System: "Nemesis: Does the Sun Have a 'Companion'?"

Shoot the Moon

Laser Ranging Retroreflectors (LRR) were left on the lunar surface by three Apollo missions. They consist of hundreds of corner-cube reflectors, which reflect incoming light beam back in the direction it came from. NASA

For years scientists have been shooting laser pulses at the Moon to determine the Earth's distance to that nearest of celestial body. They aim for reflectors left by the Apollo astronauts and unmanned Soviet missions, measure the round-trip time of the pulse, and determine the distance traveled -- you've probably seen it explained many times on NOVA and other science shows. Starting in the 1970s this technique yielded results accurate to 25-centimeters, in the mid-80s this was reduced to 2-cm, and over the next five-years a laser will refine the measurement to just 1-millimeter.

The desire to determine an exacting distance between the centers of the Earth and Moon -- about 384,000-km -- is not just for purposes of technical vanity: such precise measurements will test our understanding of gravity. Among other phenomena, these lunar distance measures will test for signs that gravity is "diluted" as the Universe expands, if gravitational attraction has any relation to the composition of a body (Which may change the answer to the silly riddle: "Which is heavier: a ton of feathers or a ton of bricks?"), and bring better understanding of how the Sun's gravitational pull affects the Earth's gravitational field. According to Tom Murphy, the University of Washington postdoctoral researcher leading the experiment, this last observation "is essentially measuring the weight of gravity, and this is the only type of project that can currently do that." The project is also a feasibility study for space-based laser-ranging experiments.

The experiment will begin in about a year and employ the 3.5-meter telescope at Apache Point, New Mexico. The laser will transmit 20-pulses per second at a peak output of 1-gigawatt; sounds like more than enough power to hit the Moon, but how will they aim it precisely enough to find a suitcase-sized bank of reflectors 100s of kilometers away? At that distance, isn't the chance of retrieving data more like a "coin flip?" Turns out our planet's atmosphere is a big factor in determining power output and pointing accuracy: it defocuses the beam on the way out to 2-km in diameter when it hits the lunar reflectors, and the reflected beam becomes 15-km wide on the return passage. With odds of 1:900-million that a single photon will hit the reflector and be detected by the telescope, you need a gigawatt of power to ensure you get some photons back. The flipside of the "coin" is that the large footprint of the laser on the Moon makes it easier to hit a reflector. Using advanced measuring techniques, Murphy expects to detect five to ten returning photons per laser pulse.

More information:
University of Washington; Jan. 14, 2002; Press Release: "UW researcher plans project to pin down moon's distance from Earth"

All in a Day's Work

Sunrise from the Space Shuttle. NASA

If you're one of those people that sense the days passing by faster-and-faster, you are quite right. Since 1992 the length of the day has been shorter than average, and the shortest day of all may happen while you're on summer vacation this year. In the last 100-years the shortest day was August 2, 2001 and the longest day happened sometime in 1912 -- the year the U.S. Congress adopted the 8-hour day for Federal workers (while everyone else worked 10-12-hour days, six-days a week!).

You'd have to be a sensitive person indeed to note these time-drifts: the August 2001 day was only short of 24-hours by 1-thousandth of a second. The longest day in 1912 was longer by a similar magnitude.

It is doubtful that any creature on Earth can sense the daily variations in the length of a day. The famous astronomer Edmond Halley first noted it through lunar observations in 1695, but mistook it for the Moon speeding up instead of the Earth slowing down. Modern astronomical methods for timing the Earth's precise rotation have included the use of transit telescopes to electronically detect the passage of an optical star, Doppler methods using satellites and fixed Earth stations, laser-ranging of retroreflectors left on the Moon, and radio interferometry using cosmic radio sources.

The effect of the jet stream has a large part to play in the seasonal variation of day-length. As Earth's heating pattern change over the course of a year, so does the jet stream. This tends to decrease rotation-rate during northern winters, which is balanced by increased rates in summer. Over the year a millisecond change in day-length may develop; these yearly trends of increased or decreased day-length may last for decades or centuries.

Because changes in Earth's mass distribution from earthquake, tides, and the jet stream can change its rotation speed, rotation measurements may improve weather, ocean, and earthquake models -- but that's in the future. A group at NASA's Jet Propulsion Laboratory (JPL) uses the global positioning system to obtain one-hundredth of a millisecond exactness in timing Earth's rotation -- for use right now. Their reason for all this accuracy: spacecraft navigation. For precise course correction maneuvers -- especially when landing on an alien world -- they need to know the orientation of the Earth in order to interpret the tracking data from the spacecraft. It's all in a day's work for JPL.

More information:
NASA/JPL; Jan. 23, 2002; Press Release: "All Days Are Not Created Equal"